GPP > NPP (NPP = GPP − R). Energy flow: sun → producers → consumers (10% law of Lindeman). Pyramids: number, biomass, energy. Inverted pyramid of biomass in ocean (algae < zooplankton momentarily). Pyramid of energy is always upright.
-- NCERT Class 12 Biology, Chapter 12, p. 207Ecosystem Productivity Decomposition
8 MCQs4 revision cards9-step worked example
Source: NCERT Ecology and EnvironmentPYQ coverage: NEET 2020, 2021, 2022, 2023, 2024, 2025, 2026Official key: NTA-verifiedLast updated: 24 Sep 2026
Ecosystem Productivity Decomposition, explained for NEET
The pyramid of energy never inverts — and NEET counts on you forgetting why.
An ecosystem's productivity starts at the producer level. Primary productivity is the total organic matter synthesised by autotrophs per unit area per unit time. Of this, Gross Primary Productivity (GPP) is the total photosynthetic output, while Net Primary Productivity (NPP) = GPP − Respiration. NPP is the biomass available to herbivores and decomposers (NCERT Class 12 Biology Chapter 12, page 207).
Decomposition breaks down dead organic matter (detritus) into simpler inorganic substances. The key steps — fragmentation, leaching, catabolism, humification, mineralisation — return nutrients to the soil. Decomposition rate depends on the chemical quality of detritus (lignin-rich detritus decomposes slowly) and climatic factors (warm, moist conditions accelerate it; low temperature and anaerobic conditions retard it).
Energy flow is unidirectional: sun → producers → herbivores → carnivores. At each transfer, roughly 10% of energy passes to the next trophic level; the rest is lost as heat through respiration (Lindeman's 10% law). This guaranteed loss means the pyramid of energy is always upright — no exceptions.
Ecological pyramids represent trophic structure graphically. The pyramid of numbers can be inverted (a single tree supports many insects). The pyramid of biomass can be inverted in aquatic ecosystems (small-bodied phytoplankton at the base, larger zooplankton above at a given instant). But the pyramid of energy is never inverted — the 10% law ensures each successive level holds less total energy.
A common mistake: claiming the pyramid of energy can be inverted just like biomass or numbers. It cannot. Energy dissipation at each level is irreversible — thermodynamics does not allow an upper trophic level to accumulate more energy than the one below it.
Can you answer these Ecosystem Productivity Decomposition MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Which of the following statements about primary productivity is correct?
Show answer and why every option is right or wrong
Answer: B. B is correct. Net Primary Productivity equals Gross Primary Productivity minus the energy used in respiration by the producers (NCERT Class 12 Biology Chapter 12, page 207).
Why A is wrong: A is wrong because GPP is the TOTAL photosynthetic output before respiratory loss is subtracted. It is NPP that accounts for respiratory loss.
Why C is wrong: C is wrong because NPP is always LESS than GPP — respiration consumes part of the gross production, so the net value cannot exceed the gross.
Why D is wrong: D is wrong because primary productivity is measured as biomass or energy per unit area per unit time (e.g., g/m²/year or kcal/m²/year), not as organism count.
MCQ 2Easy RecallPractice
Which of the following factors retards decomposition?
Show answer and why every option is right or wrong
Answer: C. C is correct. Low temperature slows microbial enzymatic activity, and anaerobic conditions limit aerobic decomposers, both retarding decomposition (NCERT Class 12 Biology, Chapter 12).
Why A is wrong: A is wrong because warm and moist conditions ACCELERATE decomposition by promoting microbial activity.
Why B is wrong: B is wrong because nitrogen-rich detritus decomposes FASTER — it is lignin-rich detritus that resists decomposition.
Why D is wrong: D is wrong because high oxygen availability supports aerobic decomposition, which ACCELERATES the process rather than retarding it.
MCQ 3Easy RecallPractice
The correct sequence of processes during decomposition is:
Show answer and why every option is right or wrong
Answer: D. D is correct. Decomposition proceeds through fragmentation (physical breakdown by detritivores), leaching (water-soluble substances drain into soil), catabolism (enzymatic degradation), humification (formation of humus), and mineralisation (release of inorganic nutrients) (NCERT Class 12 Biology, Chapter 12).
Why A is wrong: A is wrong because humification and mineralisation are the final stages of decomposition, not the starting steps.
Why B is wrong: B is wrong because leaching does not precede fragmentation — detritivores must first physically break down the detritus before water-soluble substances can leach out. Mineralisation also does not precede humification.
Why C is wrong: C is wrong because catabolism (enzymatic degradation) does not occur first — physical fragmentation by detritivores initiates the process.
MCQ 4Direct ApplicationPractice
If the Gross Primary Productivity of an ecosystem is 20,000 kcal/m²/year and the plant respiration is 8,000 kcal/m²/year, what is the Net Primary Productivity?
Show answer and why every option is right or wrong
Answer: C. C is correct. NPP = GPP − Respiration = 20,000 − 8,000 = 12,000 kcal/m²/year (NCERT Class 12 Biology Chapter 12, page 207).
Why A is wrong: A is wrong because 28,000 results from ADDING respiration to GPP instead of subtracting it. NPP = GPP − R, not GPP + R.
Why B is wrong: B is wrong because 8,000 kcal/m²/year is the respiratory loss itself, not the net productivity available to the next trophic level.
Why D is wrong: D is wrong because 20,000 is the GPP (gross value). The net productivity must subtract respiration, yielding a smaller number.
MCQ 5Direct ApplicationPractice
In an ecosystem, the energy available at the producer level is 10,000 kcal. Applying Lindeman's 10% law, how much energy is available to the secondary consumers?
Show answer and why every option is right or wrong
Answer: A. A is correct. Producers → primary consumers: 10,000 × 0.10 = 1,000 kcal. Primary consumers → secondary consumers: 1,000 × 0.10 = 100 kcal. Two successive 10% transfers yield 100 kcal at the secondary consumer level (NCERT Class 12 Biology, Chapter 12).
Why B is wrong: B is wrong because 1,000 kcal is the energy available to PRIMARY consumers (one 10% transfer). Secondary consumers are one level higher, requiring a second 10% transfer.
Why C is wrong: C is wrong because 10 kcal would be the energy at the TERTIARY consumer level (three 10% transfers from producers), not secondary consumers.
Why D is wrong: D is wrong because 10,000 kcal is the producer-level energy. No trophic level above the producers retains the full energy — the 10% law guarantees progressive loss.
MCQ 6Direct ApplicationPractice
Which of the following ecological pyramids is always upright?
Show answer and why every option is right or wrong
Answer: A. A is correct. The pyramid of energy is always upright because energy transfer between trophic levels follows the 10% law — each successive level necessarily has less total energy than the one below it (NCERT Class 12 Biology, Chapter 12).
Why B is wrong: B is wrong because the pyramid of biomass CAN be inverted in aquatic ecosystems — small-bodied, fast-reproducing phytoplankton may have less standing biomass at any instant than the larger zooplankton above them (trap: confusing the 'always upright' rule of energy pyramids with biomass pyramids).
Why C is wrong: C is wrong because the pyramid of numbers CAN be inverted — for example, a single large tree (one producer) supports thousands of insects (many primary consumers) (trap: confusing the 'always upright' rule of energy pyramids with number pyramids).
Why D is wrong: D is wrong because BOTH pyramid of numbers and pyramid of biomass can be inverted under specific ecological conditions. Only the pyramid of energy is guaranteed to be upright.
MCQ 7Concept TrapPractice
A student claims that in an ocean ecosystem, the pyramid of biomass is inverted, so the pyramid of energy must also be inverted. What is wrong with this reasoning?
Show answer and why every option is right or wrong
Answer: D. D is correct. Biomass pyramids can invert because they capture a snapshot of standing crop — small, rapidly reproducing phytoplankton may have low standing biomass at any moment despite high productivity. Energy pyramids measure total energy flow over time and are always upright because each trophic transfer dissipates ~90% as heat (NCERT Class 12 Biology, Chapter 12).
Why A is wrong: A is wrong because the pyramid of biomass in oceans IS indeed inverted (small phytoplankton biomass < larger zooplankton biomass at a given instant). The student's premise about biomass is correct; their ERROR is extending that inversion to energy (trap: claiming the pyramid of energy can be inverted).
Why B is wrong: B is wrong because inverted biomass pyramids DO occur in aquatic ecosystems (the ocean phytoplankton example is the classic case taught in NCERT). This option contradicts established ecology.
Why C is wrong: C is wrong because Lindeman's 10% law applies to ALL ecosystems — aquatic and terrestrial. The thermodynamic constraint on energy transfer is universal.
MCQ 8CalculationPractice
In a food chain, if secondary consumers have 50 kcal of energy, how much energy was originally available at the producer level? (Assume Lindeman's 10% law applies at each trophic transfer.)
Show answer and why every option is right or wrong
Answer: B. B is correct. Working backwards: secondary consumers (50 kcal) ← primary consumers (50 / 0.10 = 500 kcal) ← producers (500 / 0.10 = 5,000 kcal). Two reverse 10% steps yield 5,000 kcal at the producer level.
Why A is wrong: A is wrong because 500 kcal is the energy at the PRIMARY consumer level (one reverse step from secondary consumers). The question asks for the PRODUCER level, which requires one more reverse step.
Why C is wrong: C is wrong because 50,000 kcal would require THREE reverse 10% steps from secondary consumers, placing you one level below producers — this answer overshoots by one trophic level.
Why D is wrong: D is wrong because 500,000 kcal represents four reverse 10% steps, which would place the calculation two levels below the producer level. Only two reverse steps are needed (secondary consumer → primary consumer → producer).
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Ecosystem Productivity Decomposition: quick recall before you leave
How do you solve a Ecosystem Productivity Decomposition question? A worked example
- 1
Given
A grassland ecosystem has producers with a GPP of 40,000 kcal/m²/year. Plant respiration accounts for 50% of GPP. The food chain is: Grass → Grasshopper → Frog → Snake → Hawk.
- 2
Required
Find the energy available at the tertiary consumer level (snake).
- 3
Concept
Net primary productivity is the energy available to the first consumer level. From there, Lindeman's 10% law governs each successive trophic transfer.
- 4
Formula
NPP = GPP − Respiration
E_(n+1) = 0.10 × E_n - 5
Substitution
NPP = 40,000 − (0.50 × 40,000) = 40,000 − 20,000 = 20,000 kcal/m²/year
Energy at primary consumer (grasshopper) = 0.10 × 20,000
Energy at secondary consumer (frog) = 0.10 × (energy at grasshopper)
Energy at tertiary consumer (snake) = 0.10 × (energy at frog) - 6
Calculation
• Producers (NPP): 20,000 kcal/m²/year• Primary consumer (grasshopper): 0.10 × 20,000 = 2,000 kcal/m²/year• Secondary consumer (frog): 0.10 × 2,000 = 200 kcal/m²/year• Tertiary consumer (snake): 0.10 × 200 = 20 kcal/m²/year
Note on exact constants: The factor 0.10 in Lindeman's law and the 50% respiration fraction are problem-defined exact values; they do not limit significant figures in the answer. - 7
Final answer
Energy available at the tertiary consumer level (snake) = 20 kcal/m²/year.
- 8
Common trap
Forgetting to subtract respiration from GPP before applying the 10% law. If you start with GPP (40,000) instead of NPP (20,000), every downstream value doubles — a systematic error that gives 40 kcal instead of 20 kcal at the snake level. Always compute NPP first.
A second trap: miscounting trophic levels. Snake is the tertiary consumer (fourth trophic level), not the secondary consumer. Count: producer (T1) → grasshopper (T2, primary consumer) → frog (T3, secondary consumer) → snake (T4, tertiary consumer). - 9
Similar NEET-style question
"In an aquatic food chain, phytoplankton fix 100,000 kcal of energy. How much energy reaches the top carnivore in a five-level food chain? Also state whether the pyramid of energy for this chain is upright or inverted, and justify your answer."
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What to remember before solving Ecosystem Productivity Decomposition questions
Which Ecosystem Productivity Decomposition formulas do you need for NEET?
Lindeman's 10% law of energy transfer
Approximately 10% of energy at one trophic level is transferred to the next; rest dissipated as heat.
| Symbol | Quantity | SI Unit |
|---|---|---|
| E_n | energy at level n | kcal/m² |
Valid when
- Idealised trophic transfer
Where do students lose marks on Ecosystem Productivity Decomposition?
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Root cause: concept gap
Correction
Pyramid of ENERGY is ALWAYS upright (10% law guarantees decreasing energy at higher trophic levels). Pyramid of NUMBER and BIOMASS can be inverted (e.g. ocean: small algae < large zooplankton at one moment).
More in Ecology and Environment: 2 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.
Ecosystem Productivity Decomposition questions from past NEET papers
11 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025, 2026. Answers verified against NTA official keys.
NEET 2026
1(a), (b) and (c)
2(a) and (b) only
3(a) and (c) only
4(b) and (c) only
NTA Answer: Option 2(final)
NEET 2025
Which of the following is the unit of productivity of an Ecosystem?
1(KCal m⁻²)yr⁻¹
2gm–2
3KCal m⁻²
4KCal m⁻³
NTA Answer: Option 1(final)
NEET 2025
1Statement I is incorrect but statement II is correct
2Both statement I and statement II are correct
3Both statement I and statement II are incorrect
4Statement I is correct but statement II is incorrect
NTA Answer: Option 4(final)
NEET 2025
1Statement I is incorrect but statement II is correct
2Both statement I and statement II are correct
3Both statement I and statement II are incorrect
4Statement I is correct but statement II is incorrect
NTA Answer: Option 4(final)
NEET 2024Revised key
1(x/10) (kcal m⁻²) yr⁻¹
2x (kcal m⁻²) yr⁻¹
310x (kcal m⁻²) yr⁻¹
4(100x/3x) (kcal m⁻²) yr⁻¹
NTA Answer: Option 3(revised_final)
NEET 2023
1B, C, D only
2C, D, E only
3D, E, A only
4A, B, C only
NTA Answer: Option 4(final)
NEET 2023
1Respiratory quotient
2Respiratory loss
3Reproductive allocation
4Photosynthetically active radiation
NTA Answer: Option 2(final)
NEET 2022
1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 4(final)
NEET 2022
Detritivores breakdown detritus into smaller particles. This process is called:
1Decomposition
2Catabolism
3Fragmentation
4Humification
NTA Answer: Option 3(final)
NEET 2021
1Standing crop
2Climax
3Climax community
4Standing state
NTA Answer: Option 4(final)
NEET 2020
1There is no relationship between Gross primary productivity and Net primary productivity.
2Gross primary productivity is always less than net primary productivity.
3Gross primary productivity is always more than net primary productivity.
4Gross primary productivity and Net primary productivity are one and same.
NTA Answer: Option 3(final)
Sources
NCERT refs: Class 12 Biology Chapter 12, p.207
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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